1. Field of the Invention
This invention relates to luminescent compositions. More particularly, it relates to improved manganese-activated magnesium aluminum gallate phosphors.
2. Prior Art
Manganese activated magnesium aluminum gallate phosphors are known and are disclosed in U.S. Pat. No. 3,499,843. These phosphors when excited by 253.7 nm ultraviolet radiation are particularly applicable to electrophotographic reproduction techniques. Such phosphors emit narrow bands of visible green light. While these phosphors were satisfactory for a number of years, as methods of electrophotographic reproduction are improving new demands are being placed upon the phosphor lamp combination. This has required changes in the emission peak position, improvements in phosphors lamp brightness, increases in phosphor lamp temperature dependence stability and improvements in overall phosphor lamp processes and maintenance stability. A common phenomena observed among fluorescent lamp phosphors is the tendency for luminescent materials to degrade during the lamp manufacturing process. It is believed that this degradation is associated with the rather severe chemical activity to which the phosphor is exposed during the lehring phases of lamp manufacturing. Another associated form of degradation is generally referred to herein as maintenance degradation which is initially rapid but gradually dimensioning degradation of the lamp luminescent output as a function of lamp operating time. This maintenance degradation is generally associated with the chemical changes that have probably occurred during the time the lamp is in operation. Some experimentation has suggested a correlation between these two degradations tendencies and, thus, some interdependence between the process degradation and maintenance degradation is believed to exist. The magnesium gallate phosphors are not exception to the foregoing degradation tendencies. While some improvement has been achieved by a less severe firing procedure than is taught in the originally cited patent by a fluoride hydrolysis reaction mechanism as disclosed and taught in my co-pending patent application Ser. No. 329,651, filed Feb. 5, 1973, now U.S. Pat. No. 3,962,118 the phosphor still exhibits some degradation. It is believed, therefore, that a phosphor of the magnesium gallate type which would exhibit improved fluorescent efficiency and achieve longer lamp life and improved lamp operating maintenance would be an advancement in the art.